This invention relates to blends of a fluoroplastic and a fluoroelastomer, methods therefor, and articles made therefrom.
Fluoroplastics are unique among polymers, offering performance characteristics unobtainable with most other polymers. Commercially available fluoropolymers include polytetrafluoroethylene (PTFE), fluorinated ethylene-propylene copolymer (FEP), perfluoroalkoxy resin (PFA), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), ethylene-tetrafluoroethylene copolymer (ETFE), polyvinylidene fluoride (PVDF), and polyvinylfluoride (PVF). Some fluoroplastics such as PTFE are completely fluorinated, while others such as ETFE or PVDF are only partially fluorinated.
Fluoroplastics generally are thermoplastics, processable by conventional thermoforming techniques such as extrusion and molding, thereby facilitating their fabrication into shaped articles. Typically, they are characterized by high melting points and low glass transition temperatures, enabling them to be advantageously used over a wide temperature range, such as from well below 0.degree. C. to +260.degree. C. Other desirable properties of fluoroplastics include their excellent solvent resistance, electrical insulative properties, low coefficient of friction, low flammability, low gas permeability, and high inertness and stability. However, a limitation of many fluoroplastics is their susceptibility to stress-cracking.
Thermoplastic elastomers are materials which combine the properties of a thermoplastic and an elastomer. They can be melt processed like a thermoplastic, but, upon cooling, exhibit rubbery properties, without the need for covalent crosslinking. The development of these rubbery properties may be achieved by several methods. One method is to blend a crystalline polymer and an elastomer. Another method is to block copolymerize an elastomer with a crystalline thermoplastic, to form a block copolymer having elastomeric and crystalline domains. The copolymer may be configured such that the ends are crystalline blocks with the middle being elastomeric, or the blocks can be alternating, with many blocks per molecule.
Thermoplastic fluoroelastomers are fluorinated thermoplastic elastomers, possessing the melt processability of other fluoroplastics, along with the aforementioned desirable solvent resistance, thermal stability, electrical, and other characteristics of fluoroplastics. However, unless crosslinked, they tend to have inferior or moderate mechanical strength relative to fluoroplastics. In some instances it is possible to improve mechanical properties by blending them with a fluoroplastic; however, more often the resulting blend has inferior properties relative to the predicted properties based on the weight average of the two components -- that is, there is negative synergism.
Blending of two or more polymers is a common practice for producing a product composition having improved mechanical, rheological, and/or degradative properties compared to those of the individual polymers. It can be a very effective way to customize a composition, providing highly specific properties which may not be available in any single known polymer or which would require the time-consuming and expensive development of a hitherto unknown new polymer.
Although virtually an infinite number of different polymer blends is possible, not all polymer blends result in compositions with desirable properties. If the component polymers are incompatible, the resultant blend often will have inferior properties, particularly in the case of blends involving fluoropolymers. Normally, incompatibility is the rule, and compatibility is the exception. A rigorous and technically precise definition of a compatible blend is a blend in which the constituents are capable of forming a single phase mixture, at least in the amorphous phase, if not the crystalline phase as well. However, at a practical level a compatible blend is often defined as one which displays useful properties. In this application, this latter definition of compatibility will be used.
Disclosures of blends in which a component is a fluorinated elastomer include copending, commonly assigned U.S. application Ser. No. 07/166,954, filed Mar. 11, 1988 (European equivalent published as EP 304,487 (1989)), which discloses a blend of ETFE or PVDF with a thermoplastic fluoroelastomer. Also, Japanese patent application 52-086,442 (1977) by Daikin Kogyo KK discloses a composition comprising (a) tetrafluoroethylene/hexafluoropropene copolymer and (b) a fluorinated elastomer such as hexafluoropropene/vinylidene fluoride copolymer, tetrafluoroethylene/vinylidene fluoride/hexafluoropropene copolymer, and tetrafluoroethylene/propylene copolymer.
We have discovered that certain blends of a fluoroplastic and a fluoroelastomer are characterized by unexpected and useful properties such as high tensile strength or elongation, low modulus, increased flexibility, improved stress-crack resistance, stress-induced crystallization, and/or optical clarity. These blends are useful as wire and cable jacketing or insulation, dimensionally heat-recoverable articles, conductive polymeric compositions (when filled with conductive particles), and chemically resistant hoses.